1. Field of the Invention
This invention relates generally to methods of manufacturing noble metal (Ru, Rh, Pd, Ag, Re, Os, Ir, Pt) layers, such as for use in integrated circuits (IC) and magnetic recording media.
2. Description of the Related Art
Ruthenium metal is considered to be one of the most promising materials for capacitor electrodes of dynamic random access memories (DRAMs) that have for example Ta2O5 and/or (Ba,Sr)TiO3 (BST) dielectrics. Ruthenium is also a potential electrode material for nonvolatile ferroelectric memories. Although platinum has been widely used as an electrode material, many disadvantages are connected to that concept, For instance, it is very difficult to pattern platinum layers by etching and platinum catalyzes the dissociation of O2 into atomic oxygen. The formed oxygen thereof diffuses into the underlying barrier, which gets oxidized and forms a resistive layer. On the contrary, ruthenium films can be easily patterned by etching and they prevent oxygen diffusion by forming RuO2, which has good conductivity. Furthermore, because of its large work function, Ru is an interesting electrode material for the future CMOS transistors where SiO2 will be replaced by high-k dielectrics. Though Ru, and for the same reason Ir, are the best candidates in what comes to the oxygen diffusion barrier properties, other noble metals, like Pt and Pd, are still considered as viable candidates for the above applications. With reference to the definition of noble metal, Encyclopedia Britannica states that a noble metal is any of several metallic chemical elements that have outstanding resistance to oxidation, even at high temperatures; the grouping is not strictly defined but usually is considered to include rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold; i.e., the metals of groups VIIb, VIII, and Ib of the second and third transition series of the periodic table of elements.
In addition to electrode applications, thin Ru films find potential use in the fixture in magnetic recording technology where the ever-increasing storage densities set increasing demands on both the read and write heads and recording medium (E. E. Fullerton, Solid State Technology, September 2001, p. 87). In anti-ferromagnetically coupled recording medium, for example, a three atomic layer thick Ru film is used for separating two ferromagnetic layers. In a longer term, perpendicular recording systems (magnetization perpendicular to the film plane) are expected to replace the current in-plane or longitudinal media. To create a high performance recording media with the magnetization perpendicular to the film plane, multilayer structures composed of ultra-thin (typically less than 5 atomic layers thick) magnetic and nonmagnetic layers have been suggested. Here Ru and Pd, for example, could be employed as nonmagnetic materials. An evident challenge for these magnetic recording media applications is how to control the film thicknesses at an atomic layer level uniformly over large substrate areas.
The current metallisation technology of integrated circuits is based on electroplated copper. However, a successful electrodeposition process requires an appropriate seed layer on the diffusion barrier material. Typically copper itself, most often deposited by physical vapour deposition methods, is used as a seed layer material. Chemical methods, like chemical vapour deposition and atomic layer deposition, which could provide better step coverage for the copper seed layer, usually suffer from a poor copper to diffusion barrier adhesion. Further, a general problem related to copper seeds is their easy oxidation, which necessitates a reduction step in the beginning of the electrodeposition process. As noble metals do not oxidise easily on their surface, they can serve as good seed layers for copper electrodeposition.
Currently, Ru films are deposited either by sputtering or by chemical vapour deposition (CVD). ALD processes for depositing Ru films have not been reported, although the characteristics of thin films deposited by ALD, especially excellent step coverage (conformality), accurate and simple thickness control and large-area uniformity, are very valuable features in the above mentioned applications.
The main problem in the development work of depositing metals by ALD has been a lack of effective reducing agents, since the metal precursors applicable in ALD are typically compounds, where the metal is at a higher oxidation state (M. Ritala and M. Leskela, Atomic Layer Deposition, in Handbook of Thin Film Materials, Ed. H. S. Nalwa, Academic Press, San Diego (2001), Vol. 1, Chapter 2, p. 103). A common strategy has been to look for reducing agents that, besides reducing the metal, remove the ligands of the metal compound intact, most typically in a protonated form. The most simple of such a reaction is the process where hydrogen radicals are used as the reducing agent (A. Sherman, U.S. Pat. No. 5,916,365):
MLn(g)- greater than MLnxe2x88x92x(chemisorbed), where x=0 . . nxe2x88x921
MLnxe2x88x92x (chemisorbed)+(nxe2x88x92x)H(g)- greater than M(s)+(nxe2x88x92x)HL(g)
Other reducing agents studied for ALD include disilane, diborane, hydrogen, formaldehyde and elemental zinc. In the latter case, zinc removes the halide ligands in the form of volatile zinc halide, e.g. ZnCl2.
Aoyama et al. (Jpn. J. Appl. Phys. 38 (1999) pp. 2194-2199) investigated a CVD process for depositing ruthenium thin films for capacitor electrode purposes. They used bis-(cyclopentadienyl)ruthenium (Ru(Cp2)) as ruthenium precursor and O2 as reactive gas for decomposing Ru(Cp2) gas. The growth temperature was varied from 230 to 315xc2x0 C. and the growth rate was 25 nm/min at 315xc2x0 C. However, carbon and hydrogen were incorporated as harmful impurities in the deposited films, thus increasing the resistivity of the film. Furthermore, the general limitations of the CVD method, such as problems related to achieving good large area uniformity and accurate thickness control, still remain. In addition, it is hard to obtain good step coverage and high film purity at the same time.
The present invention aims at eliminating the problems of prior art and to provide a novel method of producing metal thin films by ALD.
In particular, it is an object of the present invention to provide processes for producing electrically conductive noble metal thin film on a substrate by atomic layer deposition methods.
It is a third object of the invention to provide methods of producing ultra-high density magnetic recording devices.
These and other objectives, together with the advantages thereof over known processes, which shall become apparent from the following specification, are accomplished by the invention as hereinafter described and claimed.
Now, we have invented novel processes for depositing metal thin films by ALD. In general, the present invention is suitable for depositing noble metal thin films, such as ruthenium, rhodium, palladium, silver, rhenium, osmium, iridium and platinum.
The resulting ALD-grown thin metal films may be utilised, for example, in IC""s, e.g. as capacitor electrodes, as gate electrodes and as seed layers for copper metallization, as well as nonmagnetic layers in magnetic media for separating ferromagnetic layers.
In the preferred embodiment of the present invention a vaporised precursor of a noble metal is pulsed into a reaction chamber, where it is contacted with the surface of a substrate placed in the reaction chamber to form a molecular layer of the metal precursor on the substrate. The reaction chamber is purged to remove excess vaporised metal precursor. Surprisingly we have now found that oxygen, in particular oxygen in molecular form, is capable of reducing noble metal compounds into elemental form. High quality metal thin films can be deposited by utilizing reactions between the metal precursor and oxygen. This is surprising, since oxygen is usually considered an oxidising source chemical in ALD and even as such an agent its reactivity is usually only modest at temperatures below 500xc2x0 C. Clearly, the reduction mechanism described herein differs from the earlier examined ALD metal processes, where the ligands are removed intact. In the processes disclosed herein, oxygen apparently burns the ligands into carbon oxides and water and, surprisingly, reduces the metal instead of forming a metal oxide, even with those metals (like Ru) that are known to have stable oxides. Thus, in the preferred embodiment the substrate comprising the adsorbed noble metal precursor is contacted with a reactant gas that comprises oxygen, preferably free oxygen and more preferably molecular oxygen. For instance, ruthenium and platinum compounds that are chemisorbed on the substrate surface can be reduced into elemental metal by using oxygen, or by providing oxygen into the reaction chamber by decomposing oxygen containing precursors, such as H2O2, into oxygen inside the reactor. Since ruthenium and platinum are noble metals, it can be concluded that oxygen could transform chemisorbed precursors of other noble metals into elemental form as well. Naturally, for metals that have less positive potential relative to the hydrogen electrode than noble metals such a mechanism cannot be expected, as these metals form more stable oxides than the noble metals.
In one embodiment, electrode layers comprising noble metals are formed in capacitor structures of integrated circuits. In a further embodiment, extremely thin films of noble metals which act as non-magnetic separation layers are used in producing ultra-high density magnetic recording devices.
More specifically, in the preferred embodiment an electrically conductive noble metal thin film on a substrate is produced by placing a substrate in a reaction chamber within a reactor, providing a vaporized noble metal precursor into the reaction chamber to form a single molecular layer of the precursor on the substrate, removing excess vaporized precursor from the reaction chamber providing a second reactant gas comprising oxygen to the reaction chamber such that the oxygen reacts with the precursor on the substrate, removing excess reactant gas and reaction by-products from the reaction chamber; and repeating until a thin film of the desired thickness is obtained.
In one embodiment a capacitor structure is produced by depositing a first insulating layer on a silicon substrate having a doped region, placing a conductive material through the insulating layer to contact the substrate, depositing a barrier layer over the exposed surface of the conductive material, depositing a first electrode layer comprising a noble metal on the barrier layer by an atomic layer deposition process, depositing a second insulating layer on the first electrode layer, and depositing a second electrode layer comprising a noble metal on the second insulator by an atomic layer deposition process.
In another embodiment an ultra-high density magnetic recording device is produced by forming a first ferromagnetic recording layer on a substrate, forming a non-magnetic layer consisting essentially of a noble metal on the first ferromagnetic recording layer by an atomic layer deposition process, and forming a second ferromagnetic recording layer on the non-magnetic layer.
A number of considerable advantages are obtained with the aid of the present invention. The well-known advantageous characteristics of ALD (accurate and simple control of film thickness, excellent step coverage, i.e. conformality, and large area uniformity) can be obtained for deposition of metal thin films. The processes of the present invention provide a method of producing high quality conductive thin films with excellent step coverage. The processes are particularly beneficial for making electrically conductive layers in structures that have high aspect ratio vias and trenches, local high elevation areas or other similar surface structures that make the surface rough. The present vapour phase processes are easily integrated into current process equipment, such as that used for the manufacture of integrated circuits (IC) or magnetic recording media.
The amount of impurities present in the metal films deposited according to the processes of the present invention is low, which is essential when aiming at high conductivity of the film. The amounts of H, C and N impurities are typically in the order of 0.1 to 0.3 at-%. The amount of residual oxygen is typically in the range of 0.3 to 0.5 at-%.
The uniformity of the films and reproducibility of the processes can surprisingly be improved by providing the substrate surface with hydroxyl groups. Such a hydroxyl group rich surface, which promotes nucleation, can be realised very easily by depositing an ultra-thin layer of metal oxide, such as Al2O3 or TiO2. In one embodiment the layer of metal oxide is deposited by ALD by using H2O and/or H2O2 as an oxygen source.